1. Field of the Invention
The present invention relates to a semiconductor laser, a method of manufacturing a semiconductor laser, an optical pickup and an optical disk system, and particularly to those suitable for application to a ridge stripe type semiconductor laser using a nitride type Group III-V compound semiconductor, and an optical pickup and an optical disk system which each use the ridge stripe type semiconductor laser as a light source.
2. Description of the Related Art
In high-density optical disk systems, a GaN semiconductor laser with an emission wavelength in a 400 nm band is used as a light source. FIG. 13 shows an example of such a GaN semiconductor laser in the related art (refer to, for example, Kiyoshi Takahashi (general editorship), Widegap Semiconductor Opto-electronic Devices, Morikita Publishing Co., Ltd. (2006), pp. 203 to 213 as Non-patent Document 1 and Japanese Patent Laid-open No. 2003-289176 as Patent Document 1). As shown in FIG. 13, in the GaN semiconductor laser, following layers are sequentially stacked over an n-type GaN substrate 101. Concretely, an n-type AlGaN clad layer 102, an n-type GaN optical waveguide layer 103, an active layer 104 of an undoped Ga1-xInxN (quantum well layer)/Ga1-yInyN (barrier layer, x>y) multiple quantum well structure, an undoped InGaN optical waveguide layer 105, an undoped AlGaN optical waveguide layer 106, a p-type AlGaN electron barrier layer 107, a p-type GaN/undoped AlGaN superlattice clad layer 108 and a p-type GaN contact layer 109. An upper part of the p-type GaN/undoped AlGaN superlattice clad layer 108 and the p-type GaN contact layer 109 are provided with ridge stripes 110 extending rectilinearly in one direction (resonator length direction). An SiO2 film 111 and an undoped Si film 112 thereon are formed to extend on the p-type GaN/undoped AlGaN superlattice clad layer 108 on both side surfaces of the ridge stripe 110 and in areas on the other sides thereof. A p-side electrode 113 is formed in ohmic contact with the p-type GaN contact layer 109 at the ridge stripe 110. The p-side electrode 113 is formed to extend on the undoped Si film 112, and to extend over the range from one resonator end face to the other resonator end face, i.e., over the whole length in the resonator length direction of the resonator. On the other hand, an n-side electrode 114 is formed in ohmic contact with the back side of the n-type GaN substrate 101.
In the GaN semiconductor laser, in order to reduce internal loss, i.e., in order to improve threshold current and slope efficiency, the distance between the well layer in the active layer 104 and the p-type AlGaN electron barrier layer 107 is set to be not less than about 100 nm, whereby the internal loss is successfully reduced from 30 cm−1 to 12 cm−1. Attendant on this, the threshold current is lowered from 38 mA to 30 mA, and the slope efficiency is enhanced from 1 W/A to 1.4 W/A.